What's wrong with my fantail goldfish?

1. Odds are that fish was not attacked. He was probably dying from ammonia poisoning (I'm speculating) and was then nibbled by the goldfish.

2. Goldfish should not be housed with tropical fish. I am assuming you meant tetra.

3. A ten gallon tank is completely overstocked by a single fancy goldfish. That one fish requires 20 gallons.

You need to upgrade for the goldfish, and identify your other fish so that you can take better care fo them.

i think of checking out the water is a robust thought. The black patches sound like an illustration of ammonia burn - that would desire to come into play in case you replaced the full tank and upset the soundness. The rubbing against gravel and facets of the tank is in many cases in all probability inflammation. probable a parasite of a few type if the fish is under pressure and the water has replaced appreciably. because of the fact he's already under pressure and displaying indications of ailment i might eliminate him from the tank. The danios nipping at his fins will merely rigidity him extra and make the subject worse. i might eliminate the two goldfish from the tank if the different is growing to be to be torpid too. once you have yet another ordinary tank interior the homestead and would desire to upload some water from that to a quarantine tank that would help very much. My wager is the full water exchange is a extensive factor in it. good success

Mostly fish when they can sense weakness in a fellow tank mate they will kill them. and yes even eat them. fish are not always strictly going to just omniverous . The general rule of thumb is to never put a fish smaller then your largest fishes mouth in the tank. I have lost many fish due to this.